Thermally stabilized acrylic fibers produced by sulfation and heating in an oxygen-containing atmosphere

ABSTRACT

A process is provided wherein the thermal stabilization of an acrylic fibrous material is accelerated by heating in an oxygencontaining atmosphere following treatment with a water-soluble persulfate compound which has been found capable of promoting the oxidative cross-linking of adjoining polymer molecules. The treatment with the water-soluble persulfate compound results in the sulfation of the acrylic polymer and renders the same amenable to oxidative cross-linking on a more expeditious basis. The resulting stabilized fibrous materials are non-burning when subjected to an ordinary match flame, and may be utilized as fire resistant textile fibers, or optionally converted to a carbonized fibrous material by heating in an inert atmosphere at a more highly elevated temperature.

llnited States Patent Riggs 51 Mar. 21, 1972 [72] lnventor: John Perry Riggs, Berkeley Heights, NJ.

[73] Assignee: Celanese Corporation, New York, NY.

[22] Filed: Mar. 9, 1970 [21] Appl. No.: 17,876

3,285,696 11/1966 Tsunoda..

3,416,874 12/1968 Robin... 8/1155 UX 3,488,151 1/1970 Noss..... ...23/209.1 F 3,497,318 2/1970 Noss ..8/1 15.5 X

OTHER PUBLlCATlONS Textile Research Journal, Nov. 1960, pages 882- 896 Primary Examiner-George F. Lesmes Assistant Examiner-J1. Wolman Attorney-Thomas J. Morgan, Charles B. Harris and Kenneth E. Macklin [5 7] ABSTRACT A process is provided wherein the thermal stabilization of an acrylic fibrous material is accelerated by heating in an oxygencontaining atmosphere following treatment with a water-soluble persulfate compound which has been found capable of promoting the oxidative cross-linking of adjoining polymer molecules. The treatment with the water-soluble persulfate compound results in the. sulfation of the acrylic polymer and renders the same amenable to oxidative cross-linking on a more expeditious basis. The resulting stabilized fibrous materials are non-burning when subjected to an ordinary match flame, and may be utilized as fire resistant textile fibers, or optionally converted to a carbonized fibrous material by heating in an inert atmosphere at a more highly elevated temperature.

21 Claims, No Drawings THERMALLY STABILIZED ACRYLIC FIBERS PRODUCED BY SULFATION AND HEATING IN AN OXYGEN-CONTAINING ATMOSPHERE BACKGROUND OF THE INVENTION In the past procedures have been proposed for the conversion of fibers formed from acrylic polymers to a modified form possessing enhanced thermal stability. Such modification has generally been accomplished by heating fibrous material in an oxygen-containing atmosphere at a moderate temperature for an extended period of time.

US. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda disclose processes for the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form. The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere involves (1) an oxidative cross-linking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups. It is generally recognized that the rate at which the stabilization reaction takes place increases with the temperature of the oxygen-containing atmosphere. However, the stabilization reaction must by necessity be conducted at relatively low temperatures (i.e., below about 300 C.), since the cyclization reaction is exothermic in nature and must be controlled if the original fibrous configuration of the material undergoing stabilization is to be preserved. Accordingly the stabilization reaction tends to be time consuming, and economically demanding because of low productivity necessitated by the excessive time requirements. Prior processes proposed to shorten the period required by the stabilization reaction include that disclosed in U.S. Pat. No. 3,416,874.

While stabilized acrylic fibrous materials may be used directly in applications where a nonburning fiber is required, demands for the same have been increasingly presented by manufacturers of carbonized fibrous materials. Carbonized fibrous materials are commonly formed by heating a stabilized acrylic fibrous material in an inert atmosphere, such as nitrogen or argon, at a more highly elevated temperature. During the carbonization reaction elements present in the fiber such as nitrogen, oxygen, and hydrogen are substantially expelled. Accordingly, the term carbonized fibrous material as used in the art commonly designates a fibrous material con sisting of at least about 90 percent carbon by weight, and generally at least about 95 percent carbon by weight. Dep'ending upon the conditions under which the carbonized fibrous material is processed, it may or may not contain graphitic carbon as determined by the characteristic X-ray diffraction pattern of graphite. See, for instance, commonly assigned U.S. Ser. No. 777,275, filed Nov. 20, 1968, of Charles M. Clarke for a preferred procedure for forming carbonized and graphitized fibrous materials from a stabilized acrylic fibrous material.

It is an object of the invention to provide an improved process for enhancing the thermal stability of an acrylic fibrous material.

It is an object of the invention to provide a process wherein the oxidative cross-linking reaction in the stabilization of an acrylic fibrous material is accelerated.

It is an object of the invention to provide a process for producing a stabilized acrylic fibrous material wherein the oxidative cross-linking reaction yields a highly uniform stabilized structure.

It is an object of the invention to provide a stabilized acrylic fibrous material which is highly amenable for utilization as a precursor in the formation of amorphous carbon or graphitic carbon fibrous materials.

It is another object of the invention to provide a stabilized acrylic fibrous material which includes bound sulfur within its molecular structure.

It is a further object of the invention to provide a process for the stabilization of acrylic fibrous materials which is readily adaptable to fibers ofvarying deniers.

These and other objects, as well as the scope, nature and utilization of the invention will be apparent from the following detailed description and appended claims.

SUMMARY OF THE INVENTION It has been found that an improved process for the stabilization of an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith comprises:

a. sulfating said acrylic fibrous material by reaction with sulfate ion-radicals derived from a water-soluble persulfate compound selected from the group consisting of potassium persulfate, sodium persulfate, and ammonium persulfate, and

b. heating said sulfated acrylic fibrous material in an oxygen-containing atmosphere at a temperature of about 200 to 290 C. until a stabilized product is formed which retains its original fibrous configuration essentially intact and which is nonbuming when subjected to an ordinary match flame.

The resulting stabilized acrylic fibrous materials commonly contain bound sulfur, and exhibit a bound oxygen content of at least about 7 percent by weight, and a carbon content of about 50 to 65 percent by weight.

DESCRIPTION OF PREFERRED EMBODIMENTS The acrylic fibrous materials undergoing stabilization in the present process may be formed by conventional solution spinning techniques (i.e., may be dry spun or wet spun), and are commonly drawn to increase their orientation. As is known in the art, dry spinning is commonly conducted by dissolving the polymer in an appropriate solvent, such as N,N- dimethyl formamide or N,N-dimethyl acetamide, and passing the solution through an opening of predetermined shape into an evaporative atmosphere (e.g., nitrogen) in which much of the solvent is evaporated. Wet spinning is commonly conducted by passing a solution of the polymer through an opening of predetermined shape into an aqueous coagulation bath.

The acrylic polymer utilized as the starting material is formed primarily of recurring acrylonitrile units. For instance, the acrylic polymer should generally contain not less than about 85 mol percent of acrylonitrile units and not more than about 15 mol percent of units derived from a monovinyl compound which is copolymerizable with acrylonitrile such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like, or a plurality of such monomers.

The preferred acrylic fibrous material is an acrylonitrile homopolymer. Preferred acrylonitrile copolymers contain at least about mol percent of acrylonitrile units and up to about 5 mol percent of one or more monovinyl units copolymerized therewith.

The acrylic fibrous materials are provided as continuous lengths and may be in a variety of physical configurations. For instance, the acrylic fibrous materials may be present in the form of continuous lengths of multifilament yarns, tows, tapes, strands, cables, or similar fibrous assemblages.

When the starting material is a continuous multifilament yarn, a twist may be imparted to the same to improve the handling characteristics. For instance, a twist of about 0.1 to 5 tpi, and preferably about 0.3 to 1.0 tpi may be utilized. Also, a false twist ma be used instead of or in addition to a real twist. Alternatively, one may select bundles of fibrous material which possess essentially no twist.

The starting material may be drawn in accordance with conventional techniques in order to improve its orientation. For instance, the starting material may be drawn by stretching while in contact with a hot shoe at a temperature of about to C. Additional representative drawing techniques are disclosed in US. Pat. Nos. 2,455,173; 2,948,581; and

3,122,412. it is recommended that the acrylic fibrous materials selected for use in the process be drawn to a single filament tenacity of at least about 3 grams per denier. If desired, however, the starting material may be more highly oriented, e.g., drawn up to a single filament tenacity of about 7.5 to 8 grams per denier, or more.

Prior to heating the acrylic fibrous material in an oxygencontaining atmosphere to accomplish the desired stabilization (as described hereafter), the fibrous material is either (1) previously sulfated by reaction with certain water-soluble persulfate compounds, or (2) is provided in intimate association with the persulfate compound so that the sulfation reaction can occur simultaneously with the oxidative cross-linking portion of the stabilization reaction.

The water-soluble persulfate compounds utilized in the present process are potassium persulfate [K S O sodium persulfate [Na S O and ammonium persulfate [(NHQ S or mixtures of the same. The preferred persulfate compound is potassium persulfate.

The sulfation of the acrylic polymer may be conducted by contacting the acrylic fibrous material with an aqueous solution containing sulfate ion-radicals derived from the watersoluble persulfate compound until substantial sulfation of the acrylic fiber has occurred. As is well known in the art, the persulfate compounds are strong electrolytes when dissolved in water, and readily dissociate into potassium, sodium, or ammonium ions, and persulfate [5 0 ions. The resulting persulfate ions dissociate thermally into sulfate ion-radicals [80, T The resulting sulfate ion-radicals possess an unpaired electron and therefore exhibit extreme reactivity characteristic of free radicals.

When carrying out the sulfation reaction in this manner, the persulfate compound may be dissolved in water to form solutions of varying molarity (i.e., about 0.001 to 5 molar). The solutions are preferably heated to about 40 to 90 C. so that the formation of the reactive sulfate ion-radicals is promoted. Such ion-radicals react with active hydrogen atoms along the main polymer chain of the acrylic polymer to accomplish sulfation. During the sulfation reaction, contact conveniently may be made between the acrylic fibrous material and the solution containing the sulfate ion-radicals by immersion or spraying. The duration of the period of contact between the acrylic fibrous material and the solution containing the sulfate ion-radicals will be influenced to some degree by the concentration of the persulfate compound dissolved in the solution, the temperature of the solution, the concentration of the sulfate ion-radicals in the solution, the degree of compaction of the acrylic fibrous material undergoing treatment, and the denier of the fibrous material. Sulfation reaction times of about 60 seconds to 10 hours, or more, may be selected.

In a further embodiment of the invention, the acrylic fibrous material and the water-soluble persulfate compound are initially provided in intimate association and are heated in a gaseous atmosphere until substantial sulfation of the acrylic polymer has occurred. Intimate association of the acrylic fibrous material and the water-soluble persulfate compound may be accomplished by contacting the fibrous material with an aqueous solution of the compound, and then drying the fibrous material whereby water is substantially expelled. Such contact can be conveniently carried out by immersion, spraying, and the like. By maintaining the solution at a temperature in the range of about 10 to 30 C. (e.g., ambient), the formation of the sulfate ion-radicals can be minimized. The concentrations of persulfate compound within such solutions can conveniently vary from about 0.001 to 5 molar. The duration of the period of contact will be influenced by the concentration of the persulfate compound within the solution, the degree of compaction of the acrylic fibrous material undergoing treatment, and the denier of the fibrous material. Contact times of about 10 minutes to 48 hours, or more, may be selected. For best results, it is recommended that contact times of at least about 1 hour be employed so that substantial diffusion of the persulfate compound into the fibrous material occurs.

The drying of the fibrous material following contact with the solution containing the persulfate compound may be conducted in any convenient manner. For instance, the fibrous material may be simply exposed to ambient conditions until water adhering thereto is substantially evaporated. For instance, drying may be conducted by exposure to a gaseous atmosphere at a temperature of about 10 to 30 C. The drying step can, of course, be expedited by exposure to a circulating gaseous atmosphere provided at an elevated temperature up to about 290 C., as will be apparent to those skilled in the art. If desired, the drying may be conveniently conducted in the same zone in which the stabilization reaction is carried out, as described hereafter. It is recommended that the acrylic fibrous material following drying be provided in intimate association with about 0.001 to 0.5 percent by weight of the persulfate compound (e.g., potassium persulfate) based upon the weight of the acrylic fibrous material.

Following the drying of the acrylic fibrous material, the sulfation reaction may be simply carried out by heating the resulting acrylic fibrous material in a gaseous atmosphere while in intimate association with the persulfate compound. Heating temperatures for the sulfation reaction range from about 40 to 290 C. The nature of the gaseous atmosphere in which such a sulfation reaction is carried out may be varied. For instance, the atmosphere may be oxygen-containing (e.g., air), or inert (e.g., nitrogen, argon, etc.). If the atmosphere is oxygen-containing and provided at a temperature of about 200 to 290 C., then the sulfation reaction can be carried out simultaneously with the oxidative cross-linking reaction of the stabilization reaction. Since, under such conditions, the sulfation reaction tends to proceed at a more rapid rate than the oxidative cross-linking reaction (described hereafter), substantial sulfation will occur immediately, which is then followed by the bulk of stabilization reaction.

The acrylic fibrous material following sulfation or while in intimate association with the persulfate compound is next exposed to an oxygen-containing atmosphere at a temperature of about 200 to 290 C. until a stabilized fibrous product is formed. In a preferred embodiment of the invention, the oxygen-containing atmosphere is air. Preferred temperatures for the oxygen-containing atmosphere are 220 to 260 C., and most preferably, 240 to 250 C.

For best results, uniform contact during the stabilization reaction with molecular oxygen throughout all portions of the acrylic fibrous materials is encouraged. Such uniform reaction conditions can best be accomplished by limiting the mass of fibrous material at any one location so that heat dissipation from within the interior of the fibrous material is not unduly impaired, and free access to molecular oxygen is provided. For instance, the acrylic fibrous material may be placed in the oxygen-containing atmosphere while wound upon a support to a limited thickness. In a preferred embodiment of the invention, the acrylic fibrous material while in a sulfated form or in intimate association with the persulfate compound is continuously passed in the direction of its length through the heated oxygen-containing atmosphere. For instance, a continuous length of the acrylic fibrous material may be passed through a circulating oven or the tube of a muffle furnace. The speed of passage through the heated oxygen-containing atmosphere will be determined by the size of the heating zone and the desired residence time.

The period of time required to complete the stabilization reaction within the oxygen-containing atmosphere is generally inversely related to the temperature of the atmosphere, and is also influenced by the denier of the acrylic fibrous material undergoing treatment. Treatment times in the oxygen-containing atmosphere accordingly commonly range from about 30 minutes to hours. Regardless of the stabilization temperature selected within the range of about 200 to 290 C., the presence of the acrylic fibrous material in sulfated form results in an accelerated oxidative cross-linking reaction for a given temperature.

The stabilized acrylic fibrous materials formed in accordance with the present process are black in appearance,

retain essentially the same fibrous configuration as the starting material, are nonburning when subjected to an ordinary match flame, commonly contain sulfur, commonly have a bound oxygen content of at least 7 percent by weight as determined by the Unterzaucher analysis, and commonly contain from about 50 to 65 percent carbon by weight.

The theory whereby the acrylic fibrous material in sulfated form serves to accelerate the stabilization reaction is considered complex and incapable of simple explanation. It is believed, however, that once reactive hydrogen atoms along the main polymer chain are replaced by sulfate groups, that these sulfate groups are capable of promoting the oxidative crosslinking portion of the stabilization reaction.

Since the oxidative cross-linking reaction is accelerated in the present process, one optionally may elect to carry out the stabilization reaction at a less severe temperature than heretofore commonly utilized. Under milder temperature conditions a more uniform stabilized fiber may be achieved in the absence of undue chain degradation.

The stabilized fibrous material resulting from the stabilization treatment of the present invention is suitable for use in applications where a fire resistant fibrous material is required. For instance, nonburning fabrics may be formed from the same. As previously indicated, the stabilized acrylic fibrous materials are particularly suited for use as intermediates in the, production of carbonized fibrous materials. Such amorphous carbon or graphitic carbon fibrous products may be incorporated in a hinder or matrix and serve as a reinforcing medium. The carbon fiber component may accordingly serve as a light weight load bearing component in high performance composite structures which find particular utility in the aerospace industry.

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples.

EXAMPLE I A continuous length of an 800 fil dry spun acrylonitrile homopolymer continuous filament ya'rn having a total denier of 1,200 was selected as the starting material. The yarn was initially dry spun from a solution of the same in N,N-dimethyl formamide solvent into an evaporative atmosphere of nitrogen. The yarn was spun as a 40 fil bundle, and plied to form the 800 fil yarn which exhibited a twist of about 0.5 tpi. The yarn was next drawn at a draw ratio of about 5:1 to a single filament tenacity of about 4 grams per denier by stretching while passing over a hot shoe at a temperature of about 160 C. for a residence time of about 0.5 second.

A sample of the yarn was wound upon a porous bobbin and immersed in a vessel containing a 0.1 molar solution of potassium persulfate provided at ambient temperature (i.e., about C.) for 16 hours. The yarn was removed from the vessel and allowed to dry at ambient conditions. The dried yarn was next placed in a circulating air oven provided at 250 C. for 90 minutes.

The resulting stabilized yarn was black in appearance, retained its original fibrous configuration essentially intact, was nonburning when subjected to an ordinary match flame, and exhibited a bound oxygen content of 7.2 percent by weight as determined by the Unterzaucher analysis.

In a control run, an identical sample of the acrylonitrile homopolymer yarn was immersed in a vessel containing water provided at ambient temperature (i.e., about 25 C.) for 16 hours, was removed from the vessel, was allowed to dry at ambient conditions, and was placed in the circulating air oven provided at 250 C. for 90 minutes. The resulting yarn was insufficiently stabilized and exhibited a bound oxygen content of only 5.7 percent by weight as determined by the Unterzaucher analysis.

EXAMPLE II Example I was repeated with the exception that the acrylonitrile homopolymer yarn was immersed in a vessel containing a 1.0 molar solution of potassium persulfate. The resulting stabilized yarn following treatment in the oxygencontaining atmosphere exhibited a bound oxygen content of 7.3 percent by weight.

EXAMPLE III Example I is repeated with the exception that sodium persulfate is substituted for potassium persulfate. Substantially similar results are achieved.

EXAMPLE lV Example I is repeated with the exception that ammonium persulfate is substituted for potassium persulfate. Substantially similar results are achieved.

EXAMPLE V A continuous length of acrylonitrile homopolymer yarn identical to that described in Example 1 is immersed for 60 minutes in a l-molar aqueous solution of potassium persulfate provided at 50 C. while wound upon a porous bobbin. During the period of immersion, substantial sulfation of the acrylic polymer occurs.

The resulting sulfated yarn is dried at ambient conditions (i.e., 25 C.), and placed in a circulating air oven provided at 250 C. for minutes. A stabilized product is formed which retains its original fibrous configuration essentially intact and which is nonburning when subjected to an ordinary match flame.

EXAMPLE VI Example V is repeated with the exception that sodium persulfate is substituted for potassium persulfate, to achieve substantially similar results.

EXAMPLE VII Example V is repeated with the exception that ammonium persulfate is substituted for potassium persulfate, to achieve substantially similar results.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.

I claim:

I. An improved process for the stabilization of an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith comprising:

a. sulfating said acrylic fibrous material by reaction with sulfate ion-radicals derived from a water-soluble persulfate compound selected from the group consisting of potassium persulfate, sodium persulfate, and ammonium persulfate, and

b. heating said sulfated acrylic fibrous material in an oxygen-containing atmosphere at a temperature of about 200 to 290 C. until a stabilized product is formed which retains its original fibrous configuration essentially intact and which is nonburning when subjected to an ordinary match flame.

2. A process according to claim 1 wherein said acrylic fibrous material is an acrylonitrile homopolymer.

3. A process according to claim 1 wherein said acrylic fibrous material is an acrylonitrile copolymer containing at least about mol percent of acrylonitrile units and up to about mol percent of one or more monovinyl units copolymerized therewith.

4. A process according to claim 1 wherein said acrylic fibrous material has been drawn to a single filament tenacity of at least about 3 grams per denier.

5. A process according to claim 1 wherein said sulfation reaction is carried out by contacting said acrylic fibrous material with an aqueous solution of said sulfate ion-radicals until substantial sulfation of the acrylic polymer has occurred.

6. A process according to claim 5 wherein said aqueous solution of sulfate ion-radicals is at a temperature of about 40 to 90 C.

7. A process according to claim 1 wherein said sulfation reaction is carried out by heating said acrylic fibrous material in a gaseous atmosphere while in intimate association with said persulfate compound until substantial sulfation of the acrylic polymer has occurred.

8. A process according to claim 1 wherein said water-soluble persulfate compound is potassium persulfate.

9. A process according to claim 1 wherein said oxygen-containing atmosphere is at a temperature of about 220 to 260 C.

10. A stabilized acrylic fibrous material containing bound sulfur, a bound oxygen content of at least about 7 percent by weight, and a carbon content of about 50 to 65 percent by weight formed in accordance with the process of claim 1.

11. An improved process for enhancing the thermal stability of an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith comprising:

a. immersing said acrylic fibrous material in an aqueous solution of potassium persulfate at a temperature of about 10 to 30 C.,

b. drying said acrylic fibrous material whereby Water is substantially expelled and said acrylic fibrous material is provided in intimate association with said potassium persulfate, and

c. heating the resulting acrylic fibrous material in an oxygen-containing atmosphere at a temperature of about 200 to 290 C. while in intimate association with said potassium persulfate until a stabilized product is formed which retains its original fibrous configuration essentially intact and which is nonburning when subjected to an ordinary match flame.

12. A process according to claim 11 wherein said acrylic fibrous material is an acrylonitrile homopolymer.

13. A process according to claim 11 wherein said acrylic fibrous material is an acrylonitrile copolymer containing at least about mol percent of acrylonitrile units and up to about 5 mol percent of one or more monovinyl units copolymerized therewith.

14. A process according to claim 11 wherein said acrylic fibrous material has been drawn to a single filament tenacity of at least about 3 grams per denier.

15. A process according to claim 11 wherein said aqueous solution of potassium persulfate has a molarity of about 0.001 to 5.

16. A process according to claim 11 wherein said acrylic fibrous material is immersed in said aqueous solution of potassium persulfate for at least about 1 hour.

17. A process according to claim 11 wherein said drying is conducting by exposure to a gaseous atmosphere at a temperature of about 10 to 30 C. i

18. A process according to claim 11 wherein said drying yields said acrylic fibrous material in intimate association with about 0.001 to 0.5 percent by weight of potassium persulfate based upon the weight of said acrylic fibrous material.

19. A process according to claim 11 wherein said oxygencontaining atmosphere is at a temperature of about 220 to 20. A process according to claim 19 wherein said oxygencontaining atmosphere is at a temperature of about 240 to 250 C.

21. A process according to claim 11 wherein said stabilized product contains bound sulfur, a bound oxygen content of at least about 7 percent by weight, and a carbon content of about 50 to 65 percent by weight. 

2. A process according to claim 1 wherein said acrylic fibrous material is an acrylonitrile homopolymer.
 3. A process according to claim 1 wherein said acrylic fibrous material is an acrylonitrile copolymer containing at least about 95 mol percent of acrylonitrile units and up to about 5 mol percent of one or more monovinyl units copolymerized therewith.
 4. A process according to claim 1 wherein said acrylic fibrous material has been drawn to a single filament tenacity of at least about 3 grams per denier.
 5. A process according to claim 1 wherein said sulfation reaction is carried out by contacting said acrylic fibrous material with an aqueous solution of said sulfate ion-radicals until substantial sulfation of the acrylic polymer has occurred.
 6. A process according to claim 5 wherein said aqueous solution of sulfate ion-radicals is at a temperature of about 40* to 90* C.
 7. A process according to claim 1 wherein said sulfation reaction is carried out by heating said acrylic fibrous material in a gaseous atmosphere while in intimate association with said persulfate compound until substantial sulfation of the acrylic polymer has occurred.
 8. A process according to claim 1 wherein said water-soluble persulfate compound is potassium persulfate.
 9. A process according to claim 1 wherein said oxygen-containing atmosphere is at a temperature of about 220* to 260* C.
 10. A stabilized acrylic fibrous material containing bound sulfur, a bound oxygen content of at least about 7 percent by weight, and a carbon content of about 50 to 65 percent by weight formed in accordance with the process of claim
 1. 11. An improved process for enhancing the thermal stability of an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith comprising: a. immersing said acrylic fibrous material in an aqueous solution of potassium persulfate at a temperature of about 10* to 30* C., b. drying said acrylic fibrous material whereby water is substantially expelled and said acrylic fibrous material is provided in intimate association with said potassium persulfate, and c. heating the resulting acrylic fibrous material in an oxygen-containing atmosphere at a temperature of about 200* to 290* C. while in intimate association with said potassium persulfate until a stabilized product is formed which retains its original fibrous configuration essentially intact and which is nonburning when subjected to an ordinary match flame.
 12. A process according to claim 11 wherein said acrylic fibrous material is an acrylonitrile homopolymer.
 13. A process according to claim 11 wherein said acrylic fibrous material is an acrylonitrile copolymer containing at least about 95 mol percent of acrylonitrile units and up to about 5 mol percent of one or more monovinyl units copolymerized therewith.
 14. A process according to claim 11 wherein said acrylic fibrous material has been drawn to a single filament tenacity of at least about 3 grams per denier.
 15. A process according to claim 11 wherein said aqueous solution of potassium persulfate has a molarity of about 0.001 to
 5. 16. A process according to claim 11 wherein said acrylic fibrous material is immersed in said aqueous solution of potassium persulfate for at least about 1 hour.
 17. A process according to claim 11 wherein said drying is conducting by exposure to a gaseous atmosphere at a temperature of about 10* to 30* C.
 18. A process according to claim 11 wherein said drying yields said acrylic fibrous material in intimate association with about 0.001 to 0.5 percent by weight of potassium persulfate based upon the weight of said acrylic fibrous material.
 19. A process according to claim 11 wherein said oxygen-containing atmosphere is at a temperature of about 220* to 260* C.
 20. A process according to claim 19 wherein said oxygen-containing atmosphere is at a temperature of about 240* to 250* C.
 21. A process according to claim 11 wherein said stabilized product contains bound sulfur, a bound oxygen content of at least about 7 percent by weight, and a carbon content of about 50 to 65 percent by weight. 